Overall Product Distribution

Although the use of cyclic enals improved the overall product distribution by suppressing the 1,4-addition pathway,33 34 a general solution remained elusive. To solve the competing reaction pathway problem or to improve the pathway that would ultimately lead to the 2H-pyran 33 involved extensive experimental modifications. Our solution eventually involved the utilization of preformed a,P-unsaturated iminium salts instead of generating them in situ.35-38... 

For heterolytic 0-0 scission following a second protonation, in which the departing oxygen atom leaves as water, electron density is withdrawn from the porphyrin moiety, and a porphyrin pi-cation radical is formed. The heterolysis/homolysis ratio and overall product distributions are thus coupled in the native enzyme systems. Various parameters such as bulk and local pH, ligation state of the metal, structure, and redox properties of porphyrin and peroxide species certainly play important roles in controlling the spin state, 0-0 bond order, and proton delivery events. 

A great amount of insight can be obtained by the computational smdy of the thermal decomposition of /3-0-4 model compounds, representing the most common linkage in lignin. While experimental work determines overall product distributions and total rates of reaction [33,37], kinetic parameters of individual reactions and details of substiment effects on equilibrium and transition state structures are difficult to obtain... 

Competitive hydrogen abstraction reactions by phenoxy and benzyl radicals on the a-or )8-carbon of PPE are the core reactions of the radical chain propagation (Scheme 7.2) in the thermal degradation of PPE and are the only reactions needed in the analytical kinetic model derived in Section 7.3.2.3 to calculate overall a/)8-selectivities. Because hydrogen abstractions are the rate-determining steps, effects of substituents on hydrogen abstraction have the most impact on overall product distributions and pyrolysis rates. 

Overall Product Distributions. Depending on the catalyst t5 pe and on the process conditions, the selectivity of methane can vary from about 2 to 100% whereas that of long-chain waxes can vary from 0% to more than 70% ((7), chapter 3, (20)). Despite the wide distribution of products, there is nevertheless always a clear interrelationship between the individual products, as is illustrated in Figures 5 and 6 ((7), chapter 3) The accepted explanation is that the FT reaction involves the stepwise chain growth of monomers. The actual structure of the monomers is disputable (see the section FT Surface Reactions and Mechanisms ). 

As is mentioned in the section Overall Product Distributions, the C2 compounds and methane invariably do not fall on the ASF plots. The C2 falls below the line, and the Ci, particularly so for Co catalysts, falls above the line. (The ASF deviations have been discussed in chapters 3 and 8 of Reference 7). An isolated CH2 monomer (see Fig. 12a) is likely to be rapidly hydrogenated and this could explain the higher than expected methane 3delds. One of the reasons for the low levels of C2 compounds could be their reincorporation into growing chains. Alternatively, if the C2 surface specie is taken as the adsorbed ethylene, the addition of another CH2 could occur on either side of it, which would increase the probability of chain growth (see Fig. 12b). For C3 and higher adsorbed alkenes, however, a CH2 group, for steric hindrance reasons, is more likely to attach to the open side of the adsorbed alkene (see Fig. 12c). 

Knowledge of the mechanisms of action of most solid catalysts remains extremely limited. Often the overall product distribution of a catalytic reaction is known, but the nature of the catalyst-reactant interactions remains obscure. In a few cases, a more detailed model can be advanced, but the models are modest in comparison with structural and mechanistic details that have been developed in molecular chemistry. The difficulty is that by comparison with strictly molecular systems, the surfaces of solid catalysts are extremely complicated, and the structures of the so-called "active sites" are almost always unknown at an atomic level. 

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